1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic printer or a photocopier. More particularly, the present invention relates to a developing unit for visualizing an electrostatic latent image formed on a photoconductive medium by a developer, and especially, to a method for providing stable regulation of an amount of developer transferred to the photoconductive medium and a developing unit for implementing the method.
2. Description of the Related Art
Generally, electrophotographic image forming apparatuses such as printers and photocopiers form an electrostatic latent image by electrifying a photosensitive means such as a photoconductive drum or a photoconductive belt and develop the carried image into a visible image by the developer and transfer the electrostatic latent image to a printing medium, such as paper, to develop the visible image. Accordingly, a desired image is output.
Types of developing systems used in electrophotographic image forming apparatuses include one-property developing systems, which use only a powder toner for printing and two-property developing systems, which use a developer comprising a carrier having a ferrite component and toner particles.
Such a two-property developing system is getting widely adopted in the high-speed printer field due to its fast printing, the long lifespan of the consumable parts and its low maintenance cost.
As shown in FIG. 1, a conventional electrophotographic image forming apparatus generally includes a developing unit for supplying a developer to an electrostatic latent image formed on a photoconductive medium, thereby converting the electrostatic latent image into a visible image.
FIG. 1 schematically shows a developing unit for an electrophotographic image forming apparatus using the conventional two-property developer.
Referring to FIG. 1, the conventional developing unit comprises a developer receptacle 11 storing a developer 1, a developing sleeve 14a, a magnet roller 14b, a developer-supply roller 12 and 13, and a regulating blade 15.
The developing sleeve 14a is rotatably mounted in the developer receptacle 11. The magnet roller 14a is fixed in the developing sleeve 14a. The developer-supply rollers 12 and 13 such as agitating screws transfers the developer 1 onto a surface of the developing sleeve 14a. The regulating blade 15 restricts the thickness of the developer supplied to the surface of the developing sleeve 14a to a thin layer. A photoconductive drum 16 serves as an image carrier. A developing roller 14 comprises the developing sleeve 14a and the magnet roller 14b that are engaged with each other.
In the conventional developing unit as the above, the developer 1, supplied to an N-pole by rotation of the developing sleeve 14a, is restricted by the regulating blade 15 to a thin layer on the developing sleeve 14a while being transferred from an S2-pole to an N1-pole. When the thin layer of developer is transferred to an S1-pole, which is a main pole of developing, the developer is drawn by the magnetism to develop the electrostatic latent image formed on the photoconductive drum 16. The developer on the developing sleeve 14a is then recovered into the developing receptacle 11 by repulsive magnetic fields of N2 and N3 poles.
According to the conventional developing unit using the two-property developer, when magnetism in a normal direction is much stronger than magnetism in a tangential direction, the amount of developer transfer is highly influenced by the magnetic state of the surface of the developing sleeve 14a. This is because the capability of the developing sleeve 14a to transfer the developer heavily depends on the surface configuration of the developing sleeve 14a or an abrasion state of the developer surface.
Also, as the amount of the developer held by the developing sleeve 14a increases, the developer deteriorates due to excessive friction thereof.
The transferring capability of the developing sleeve 14a may be improved by the tangential magnetism if the tangential magnetism is stronger than the normal magnetism. Accordingly, variation in the amount of developer, which is due to the surface configuration of the developing sleeve 14a and the abrasion state of the developer surface, can be restrained.
However, because the amount of developer at the developing sleeve 14a decreases, it can not generate enough frictional electrification of the developer. Therefore, a background image may appear, or the developing characteristic of a solid image may deteriorate.
Therefore, the developing unit for the image forming apparatus using the two-property developer is required to restrain deterioration of the toner and carrier as much as possible in order to obtain a high-quality image for a long time and to stabilize the amount of developer transferred to the photoconductive medium.
As a suggestion for satisfying the above requirements, Japanese Patent Laid-open No. 2003-15422 discloses a method for stabilizing the amount of the developer transferred by locating a developer-regulating position where a difference between the vertical magnetic flux density and the horizontal magnetic flux density is within a range of approximately 5 mT, as shown in FIG. 2.
However, the force which transfers the developer is based on the magnetic attraction force which draws the developer to the developing sleeve 14a. Therefore, a moving range of the developer is hard to precisely regulate using only the vertical and horizontal magnetic flux densities as shown in FIG. 2 and as disclosed in the Japanese Patent Laid-open No. 2003-15422.
Meanwhile, to insure a long lifespan for the developer, it is preferable to reduce stress on the developer-regulating position, which again depends on the low magnetic attraction force.
Magnetic poles adjacent to the developer-regulating position are the main cause of the deterioration of the developer. The magnetism may be stronger when more magnetic poles precede the developer-regulating position. However, this shortens the lifespan of the developer. This is because the magnetic attraction force increases at the positions preceding the developer-regulating position.
Thus, a low magnetic attraction force at a position preceding the developer-regulating position is an essential condition for low stress on and a longer lifespan for the developer. If a peak value of the magnetic attraction force precedes the developer-regulating position, the developer past the developer-regulating position tends to return to the position of the peak magnetic attraction force and therefore, transfer of the developer becomes unstable.
To summarize, it is an effective condition for stabilizing the transfer amount of the developer that the peak magnetic attraction force does not exist at position preceding the developer-regulating position.